This invention relates to use of laser technology to measure thickness of materials, particularly glass, and more particularly molten glass.
In the glass industry there is a need for measurement of the thickness of glass plate during various stages of manufacture.
Several U.S. Patents disclose thickness measurement methods such as:
(a) measurement of transparent glass or plastic plates by differentiating between the convergence and divergence of an interference fringe pattern created by directing a non-parallel pencil or wedge of rays onto the plate;
(b) use of an interferometer sensor for measuring distance changes of a small surface by splitting a light beam into a measurement beam and a reference beam having two different polarizations, directing the measurement beam through a retarder mounted between the two surfaces and detecting the differences in the optical paths of the measuring and reference beams;
(c) use of an adjustable interferometer to give unity fringe visibility to introduce a controlled prescribed relative phase shift between a reference wavefront and a wavefront from the optics being tested, which permit analysis of the interference fringe pattern using standard phase extraction algorithms;
(d) use of an apparatus (which includes a means for directing a light beam onto thin optical membranes, means for varying the angle of incidence of the light beam upon the membrane, and a means for detecting the angles of incidence of the light beam on the membrane) for measuring the optical thickness or index of refraction of thin optical membranes;
(e) projecting a light beam through an insulating multiple glass sheet insulating unit to determine the thickness of the insulated glass by measuring the size of a beam spot on a target; and
(f) use of a thickness measuring gauge for insulating glass which includes a sighting member with a front sight, a rear sight and a sight tube through which the user takes an angled sighting of the insulating glass with the device in contact with the near side surface of the glass. A separate reflective member is situated on the far side of the insulating glass to reflect into view the image of a movable, adjustable target member which reflects the light.
Optical methods are dependent upon the optical transparency of the material to be tested and would be sensitive to many of the harsh conditions present within the high temperature environment of the glass manufacturing process. Optical interference methods are adversely impacted by the bright, black body light emission produced by molten glass.
Ultrasound methods would be sensitive to thermally induced turbulence within the measurement region and process-produced vibrations within the material. Normally ultrasound methods require direct contact of the sound generating and detecting instrumentation with the material surface in order to overcome sound transmission limitations due to the large differences between impedence of air and impedence of the materials being measured by ultrasound.
Thus there is still a need for ways of measuring the thickness of materials such as glass in molten states during manufacture with a truly remote sensing system in which there is no necessity for placing measurement components in contact with or in the immediate proximity of the material being tested for thickness.
Therefore, it is an object of this invention to provide a method and apparatus for measuring the thickness of materials using laser technology.
It is another object of this invention to provide a method and apparatus for measuring the thickness of materials such as glass in molten states.
It is a further object of this invention to provide a method and apparatus for measuring the thickness of materials without a necessity of placing measurement components in contact with or in the immediate proximity of the materials being tested.
It is yet another object of this invention to provide a method and apparatus for measuring the thickness of opaque, partially opaque or reflective materials.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. The claims are intended to cover all changes and modifications within the spirit and scope thereof.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, there has been invented a method of measuring material thickness comprising:
(a) contacting a surface of a material to be measured with a high intensity, short duration laser pulse at a light wavelength which heats the area of contact with the material, thereby creating a compressive acoustical pulse within the material;
(b) timing the intervals between deflections in the contacted surface caused by reverberation of the acoustical pulse between the contacted surface and the opposite surface of the material; and
(c) calculating the thickness of the material by multiplying the speed of sound within the material to be measured by one half the length of the time intervals between deflections of the contacted surface.
The material thickness is proportional to the time intervals between deflections in the contacted surface because the length of time it takes for a compressive acoustic wave to travel from the contacted surface to the opposite surface and back again to the contacted surface is proportional to the thickness of the material.
The time interval or delay between deflections in the contacted surface can be measured by detection of changes in angle of reflection of a continuous beam of light reflected from the contacted surface of the material to be measured.
Alternatively, after contacting the surface of the material with laser energy to produce a compressive acoustic wave through the material, deflections of the opposite surface of the material can be detected and monitored using such means as a pressure transducer and an oscilloscope. The thickness of the material can be calculated using the time it takes for the acoustical pulse to travel through the material and the speed at which an acoustical pulse travels in the particular type of material being measured.
An apparatus for carrying out the method of the invention has a means for contacting the surface of a material to be measured with a short duration laser pulse at a wavelength which heats the area of contact with the material, thereby creating an acoustical pulse within the material; and a means for monitoring either the reverberation of acoustical pulses between the two surfaces of the material or for monitoring travel of acoustical pulses from one surface of the material to the opposite surface.